The present invention relates to a compressed metal oxide composition for use in removing contaminants, including sulfur compounds, from fluids, and a method for making such compressed metal oxide composition. More preferably, the present invention relates to a compressed iron oxide composition, with the binder preferably being a water insoluble composition.
It is well known to use metal oxides, particularly iron oxide (Fx Oy) in a reactor bed to remove contaminants, typically sulfur compounds, especially hydrogen sulfide (H2S), from fluids, typically gas streams. Sulfur compounds are removed from fluids because they are known contaminants, which potentially make gas streams or other fluids unsalable. Gas that contains too much sulfur is known as sour gas. As such, in the gas industry, as well as related industries, it is considered necessary to remove sulfur compounds from fluids, including gas. Note that these fluids are typically devoid of oxygen. (It is known that oxygen can increase reactivity between a metal oxide composition and contaminants.) For this reason, there is a need for products that remove sulfur compounds from fluids efficiently and cost effectively. It is further desired to have a method or composition that does not require the inclusion of activating agents, such as oxygen. Unfortunately, most commercially available iron oxide compositions (the most frequently used metal oxide material in removing sulfur) that operate at ambient conditions and are generally non-activated, hold an amount of sulfur equal to at most 20% by weight of the total iron oxide composition. More typically, iron oxide material (like that compressed in the present process) will hold, on average, 0.12 lbs. of H2S/lb. of iron oxide. (The percent held is dependent, in part, on the particular species of iron oxide used.) Increased H2S absorption capacity for iron oxide compositions, above 20%, typically requires the addition of a caustic or oxygen to the feed gas, which is dangerous and potentially difficult, especially at high pressures. This is problematic because approximately 80% of the total metal oxide product is unused. For this reason, frequent replacement of the metal oxide is required. Consequently, it is desired to increase the percent by weight of sulfur held by the total metal oxide product.
Sulfur removal on a level that treats up to millions of cubic feet of gas per day or on an industrial scale, typically requires the use of large reactor beds filled with the iron oxide media or product. Typically, this media is comprised of iron oxide and a carrier such as montmorillonite or wood chips. In order to eliminate frequent change-outs, which is the replacement of spent metal oxide media (media that no longer has suitable reactivity with sulfur) with new metal oxide media, large or numerous reactor beds are used. These reactor beds will either be very tall, 10 feet or higher, or multiple reactors will be lined up in succession so that a plurality of reactor beds will be used. If the reactor beds are too small or few, the metal oxide will be spent too fast. This is because when treating large volumes of gas or other fluids, the metal oxide found in the metal oxide media will be rapidly reacted. In order to have a sufficient bed life so that frequent changings of the metal or iron oxide media is not required, large amounts of metal oxide must be used. This is disadvantageous for a couple of reasons. First, the amount of sulfur held by the metal or iron oxide composition is low relative to the total weight of the product used. In order to increase efficiency, it is desired to have a product that holds a greater percentage of reacted sulfur per pound of total product. Secondly, the amount of area required to remove sulfur can increase costs. It is desired to have the option to decrease the total area required to remove H2S. In other words, it is desired to hold a greater amount of sulfur with a decreased amount of metal oxide composition.
One way to increase the amount of sulfur held in a reactor vessel is to pelletize or compress the metal oxide. The amount of sulfur held by the metal oxide composition is increased because there is more available metal oxide in the vessel. Normally, metal oxide is placed on a carrier, with the carrier comprising approximately 80% by weight of the metal oxide composition. Conversely, a pellet is typically comprised of an amount of binder equal to from about 1% to about 20% by weight of the pelletized mixture. As can be seen, the amount of metal oxide is significantly increased. The binders that have been used to form the pelletized iron oxide particles include cement, bentonite, and similar compositions, especially inorganic compositions. The pelletized particles made from these binders, however, have suffered from a problem in that it appears that the efficiencies have been lowered and that the reactivity of the metal oxides has been decreased. In particular, the amount of sulfur held is not significantly increased over the amount of sulfur held by the same species of metal oxide particle on a carrier. For this reason, prior attempts to pelletize metal oxide have been considered unsuccessful because of inadequate sulfur reactivity, in particular, holding capacity. Thus, it is necessary to find a binder that allows for sufficient binding of the metal or iron oxide particles without lowering the reactivity or efficiency with which the sulfur compounds are removed. More particularly, it is necessary to find a binder that permits the metal oxide to hold a greater amount of sulfur, in particular, H2S, without the presence of a caustic or the addition of oxygen in some form.
As stated, it has been known to pelletize metal oxides for use in removing sulfur compounds from fluids. In particular, U.S. Pat. No. 4,732,888, invented by Jha et al. discloses a zinc ferrite pellet for use in hot coal gas desulfurization. The patent discloses a composition comprised of zinc and iron oxide compressed together with inorganic and organic binders, and a small amount of activator. Inorganic binders include bentonite, kaolin, and Portland Cement. The organic binders include starch, methylcellulose, and molasses. The pellets have a very specific product design because they are used in beds having temperatures of at least 650xc2x0 C. Because of the high temperatures, the organic binders dissipate leaving pellets that are fragmented and porous. Thus, the organic binders are included for the specific purpose of holding the pellets together, initially, and then dissipating so as to create greater porosity. While this design is outstanding for use in high temperature coal desulfurization processes, it does not provide for sufficient removal at ambient conditions. As implied, it has been observed that inorganic binders decrease the amount of sulfur removed by pelletized metal oxides. As a result, insufficient removal of sulfur will likely occur at ambient or near ambient conditions when inorganic binders are used to bind the pellets together. It should also be noted, that it has previously been believed that organic binders were unacceptable for forming pellets used at ambient conditions, because the organic binders generally do not provide for a pellet that has sufficient crush strength, or there is insufficient reactivity, or the use of the binders creates a pellet that is cost prohibitive.
The present invention relates to a compressed metal oxide composition used in the removal of contaminants, preferably sulfur compounds, from fluids, and methods related thereto. The compressed metal oxide composition will be comprised of an amount of metal oxide equal to at least 80% by weight of the compressed metal oxide composition. The compressed metal oxide composition will retain an average amount of sulfur equal to at least 10% by weight of the compressed metal oxide composition and, more preferably, an amount of sulfur equal to at least 30% by weight of the compressed metal oxide composition. Importantly, the compressed metal oxide composition will hold a greater amount of sulfur than if the particular metal oxide species used to form the compressed metal oxide composition was used in association with a carrier. The compressed metal oxide composition is further advantageous because it will sufficiently remove sulfur at temperatures of less than 200xc2x0 C. and, even more advantageously, at ambient conditions.
The compressed metal oxide composition will be comprised of an amount of metal oxide, preferably in powder form or having a small particle size, and a binder. The metal oxide will have a particle size ranging between about 0.1 microns and about 100 microns, which means that the metal oxide will be similar to dust, also known as fines. Any of a variety of metal oxides, which are reactive with sulfur compounds may be used to form the compressed metal oxide composition. Most preferably, the metal oxide will be of the formula MexOy, with Me selected from the group consisting of row 4, 5, 6, and 7 metals, with x equal to between 1 and 3, and y equal to between 1 and 4. It is more preferred if the metal oxide is an iron, zinc, or manganese oxide composition, as these metal oxides have been known to readily react with sulfur compounds. In particular, iron oxide of the formula FeaOb will be preferred with a equal to between 1 and 3, and b equal to between 1 and 4. As such, compositions that include iron oxides of the formula Fe3O4 are most preferred.
Any of a variety of organic binders may be used to hold the metal oxide particles together to thereby form the compressed metal oxide composition. The binder selected must permit the metal oxide to be reactive with the sulfur compounds. As such, it has been determined that suitable binders include starch compositions, carboxymethylcellulose, and mixtures thereof. Other suitable binders include cellulose compositions. Water insoluble binders are preferred, with water insoluble cellulose compositions being the most preferred binders. Additionally, lignin, bentonite, and lignosulfonate may also be used as binders. The binder can be added to the metal oxide in an amount equal to between 0.5% and 20% by weight of the metal oxide and, more preferably, in an amount equal to between 0.5% and 5% by weight of the metal oxide.
The method of the present invention involves combining the binder with the metal oxide particles and thoroughly mixing the two constituents. Once the two constituents are mixed, it is necessary to compress the mixture so as to form the compressed metal oxide composition. The techniques used to compress the constituents to form the compressed metal oxide composition can be any of a variety of techniques or devices, including extrusion or compaction. Any compression device or method can be used, as long as the compressed metal oxide composition is suitably formed. The current invention uses extrusion or compaction to compress the metal oxide mixture. The constituents can be passed through an extruder to form a compressed metal oxide composition. Conversely, the constituents can be placed in a compactor to form a compressed metal oxide composition. A compactor is a device having at least two symmetrical wheels, which turn in opposite directions, thereby compacting the metal oxide composition between them. The use of either an extruder or a compactor has been found to produce compositions which have sufficient reactivity with contaminants, especially sulfur compounds. Any of a variety of compactor or extrusion devices may be used. The compressed metal oxide composition particles may be formed into the desired particle size at the time of formation or may be formed and then broken into the desired size. At least 90% of the compressed metal oxide composition particles shall have a final particle size equal to between about 0.1 mm to about 200 mm. It is not possible to have 100% of the compressed metal oxide composition particles within this range because the smaller end of the range will include compressed metal oxide composition particles which are powder or fines and some of these fines will enter into the final product. Preferably, the final particle size is equal to between about 0.1 mm and 20 mm. More preferably, the final particle size is equal to between about 0.5 mm to about 5 mm.
Preferably, once the metal oxide mixture has been compressed, it is broken apart so as to form the compressed metal oxide composition particles of the desired size. The smaller final particle size increases the amount of hydrogen sulfide which the compressed metal oxide composition particles are able to absorb. The broken apart compressed metal oxide composition particles are then processed through a screener for granular classification to ensure at least 90% of the particles have the correct particle size. Any oversize particles are discharged into a hammer mill, and then sent back to the screener. The fines, meanwhile, are recycled back into a batch of the compressed metal oxide composition.
The present invention is advantageous for a number of reasons. In particular, the compressed metal oxide composition particles allow for a product that can be used in a reactor bed, whereby the product reacts with a greater amount of sulfur so that a greater amount of sulfur is found in the reactor bed. This is desirable because a lesser amount of overall space can be used and fewer reactor vessel change-outs are required. The present invention is also advantageous because it demonstrates that compressed metal oxide composition particles can be formed that have sufficient reactivity with sulfur. This means that the particles are suitable for commercial use, unlike many other known pelletized metal oxide compositions. Finally, the present invention is advantageous over previous inventions which form pellets because the particles of the present invention are formed by breaking apart the composition to form smaller particle sizes which allow for more of the metal oxide to be available to react with the H2S, thereby leading to increased absorption of the H2S when using these particles.